Q: What happens to a 2×4 stud when it gets wet?
A: It dries.
Q: What happens if that 2×4 is wrapped in plastic after it gets wet?
A: It doesn’t dry.
Q: What happens to a wet 2×4 that can’t dry?
A: It grows mold and rots.
Q: What happens to the typical American house that’s built in a four- to six-month cycle?
A: The moisture in the framing lumber dries after a few months of heating and cooling, and the house “settles.” This is normal, but it doesn’t need to be pronounced.
To get a handle on what's happening here, let’s first talk about hygroscopy, a critical property of wood. Hygroscopy is the ability of a substance to attract and hold water molecules from the surrounding environment. This is achieved through either absorption (liquid dissolved into the material) or adsorption (liquid adheres to surfaces of material). As water molecules become "suspended" between the wood’s molecules, the wood is physically changed; namely it increases in volume, and this swelling (and subsequent drying) is what pushes a house around and is what people perceive as "settling."
Most of the lumber used in my homes is kiln dried, meaning it should arrive at the jobsite at a moisture content of 12% to 14%. Non-KD lumber or pressure-treated lumber generally is 15% to 19% moisture content. That 19% is a critical number; the general consensus among building-science experts is that above 19% MC, mold spores are activated and the rot processes start. Wood can absorb water up to 28% to 30% MC and becomes saturated at that point.
When house lumber is “wet” and construction continues through drywall to the finish stage, problems often arise. The most common problems include
- Drywall cracks (particularly above windows and doors)
- Nail “pops”
- Floor squeaks and stair squeaks
- In a worst case scenario, plumbing stacks can be pushed around, leading to water leaks.
To avoid problems, I use a simple moisture meter to check framing before drywall. I like pin-based meters. The picture above shows the Extech model I use, which works well. My goal is to verify that there is zero lumber with a MC above 19%. This is a hard rule. If you are getting readings on your moisture meter above 19%, then cancel with your drywall crew until all of the wood in the frame is below that 19% threshold.
Testing is critical prior to hanging rock. As I said before, all readings must be less than 19%, but my standard is that all readings be leass than 13% to 15% for KD lumber and less than 17% for green pressure-treated lumber. Once a home is completed and the heating/cooling equipment runs for a year, it’s typical to see those studs reach equilibrium MC around 10% to 12%. That’s why I really want to see MCs below 15%. A small degree of drying is expected, but when there is more than 5% MC loss, the shrinkage of that lumber will appear as flaws in your finishes.
For more detail, see my articles in JLC on Drying Wet Framing and Conditioning Homes.
This article originally appeared on MattRisinger.com. Be sure to check out Matt's blog, which has been revamped and is now categorized by topic area, making it easier to find the wealth of building knowledge there.